1. Field of the Invention
The present invention relates to a pickup structure for optically reading recorded data from a recording medium.
2. Description of the Prior Art
In an optical disk player or the like, as shown in FIG. 1A, a light-emitting-receiving composite element called a laser coupler 11 is used in an optical pickup. In the laser coupler 11, photodiodes 13 and 14 are formed on an Si substrate 12, and a prism 15 is fixed on the photodiodes 13 and 14 by an adhesive.
A submount 16 serving as another Si substrate is fixed on the Si substrate 12 by soldering or the like. A photodiode 17 is formed on the submount 16, and a laser diode 21 is fixed on the submount 16 by soldering or the like.
In order to reproduce data recorded on a optical disk 22 using the laser coupler 11, part of the light 23 emitted from the laser diode 21 is reflected on a surface 15a of the prism 15, transmitted through an objective lens 24, and incident on the optical disk 22.
The part of the light 23 which is reflected by the optical disk 22, is transmitted through the objective lens 24, and incident on the surface 15a of the prism 15 and is refracted and enters into the prism 15. The first half of the light 23 which enters into the prism 15 is incident on the photodiode 13, and the second half is reflected by the photodiode 13 and is totally reflected by a surface 15b of the prism 15, and is incident on the photodiode 14.
The photodiode 14, as shown in FIG. 1B, is divided into three parts A, B and C, and the photodiode 13, as shown in FIG. 1C, is divided into three parts D, E and F. An output obtained from these photodiodes 13 and 14 and expressed by: EQU (A+C+E)-(B+D+F)
is used as a focus error signal.
Therefore, as described above, 50% of the light 23 which enters into the prism 15 through the surface 15a must be incident on the photodiode 13, and the remaining 50% must be reflected by the photodiode 13.
For this reason, in the conventional pickup used in an unpolarization optical system recording scheme such as an optical disk, as shown in FIG. 3, an optical semitransparent film 25 (half mirror) having a reflectance of 50% is deposited on a portion of the prism 15 opposite to the photodiode 13.
Note that an SiO.sub.2 film 26 serving as a protection film is formed on the surface of the Si substrate 12, and an adhesive layer 27 is interposed between the SiO.sub.2 film 26 and the optical semitransparent film 25 on the photodiode 13 as shown in FIG. 4. Furthermore, the photodiode 17 receives the light 23 emitted from the rear surface of the laser diode 21 to perform automatic power control.
The refractive indices of the Si substrate 12, the SiO.sub.2 film 26 and the adhesive layer 27 are about 3.5, about 1.5 and about 1.45, respectively, with respect to light having a wavelength of 780 nm. For this reason, about 16% of the incident light is reflected by the interface between the Si substrate 12 and the SiO.sub.2 film 26 which have a large refractive index difference, and as shown in FIG. 4, the light 23 is reflected in a multiple form between the interface and the optical semitransparent film 25 which causes multiple interference to occur.
When the light 23 serving as convergent light having incident angles changed according to incident positions is incident on the portion where the multiple interference occurs, bright and dark portions are formed according to the incident angles, and fringes shown in FIG. 5 are formed. Since the fringes are functions of the incident angles and wavelengths, when the wavelength of the laser diode 21 is varied by a change in temperature, the fringes move.
The movement of the fringes adversely affect the variable temperature characteristics of the laser coupler 11, and an optical disk player or the like using the laser coupler 11 is defective under the standards of currently available optical disk players and the like.
As shown in the prior art FIG. 6, the optical semitransparent film 25 is not formed, and an SiN film 31 having a refractive index of about 2.0 is formed on a portion of the SiO.sub.2 film 26 on the photodiode 13. In this manner, a structure for obtaining the function of an optical semitransparent film with four layers consisting of the adhesive layer 27, the SiN film 31, the SiO.sub.2 film 26 and the Si substrate 12 is also considered.
In this structure of FIG. 6, however, the maximum reflectance is about 40%, and a ratio of the amount of light incident on the photodiode 13 to that on the photodiode 14 cannot be set to be 1:1. This ratio of the light amounts may be electrically corrected. However, in this case, noise in the photodiodes 13 and 14 cannot be differentially reduced. That is, an optical optimal point and a signal optimal point do not coincide with each other, thereby degrading the playability of an optical disk player.
In addition, in order to obtain the structure in FIG. 6, each of the products of the thicknesses and refracting indices of the SiN film 31 and the SiO.sub.2 film 26 must be set to be .lambda./4 of the light 23. However, the SiN film 31 and the SiO.sub.2 film 26 each have a high-precision film thickness and cannot be easily formed in mass production.
Therefore, the above problem cannot be easily solved by the prior art. Although an unpolarization optical system recording scheme is applied to a laser coupler used for an optical disk in which the above laser diode, a microprism and the like are integrally arranged, this recording scheme can be used for only reading data.
As a recording medium capable of repetitively recording and erasing data, an optomagnetic disk serving as a polarization optical system recording scheme is known.
A basic optical system of the above optomagnetic disk is shown in FIG. 2. Multi-mode laser light emitted from a semiconductor laser 46 passes through a collimating lens 45 and a beam shaping prism 44 and is incident on a special beam splitter 43. In the special beam splitter 43, 1 the linearly polarized component of the laser light is highly purified, and 2 weak signal light is separated from the laser light.
A light-receiving optical system splits light reflected by the special beam splitter 43 into light components to a data signal detecting system and a focus tracking control signal detecting system, and the light component incident on the data signal detecting system is guided to a differential optical system.
In addition, the light component incident on the focus tracking control signal detecting system is guided to a focus error signal detecting optical system and to a tracking error signal detecting optical system.
As a conventional pickup for recording and reading data on/from the above optomagnetic disk, a pickup obtained by assembling parts such as a polarization beam splitter and fixing them with an adhesive or the like is known.
The conventional pickup obtained by assembling these parts does not have a small size and thin thickness.
A pickup used in a polarization optical system recording scheme must have a structure in which a laser coupler obtained by integrating a laser diode, a microprism and the like is formed to equally split light which enters into the prism into light components to two or more photodiodes.